Lightning includes electrical discharges within a cloud, intracloud (IC) discharges, and cloud to ground (CG) discharges (total lighting. Lightning occurs when electrical fields within a cloud intensify as particles of opposite polarity collect at differing regions within the cloud. Typically, the electric field forms as a result of strong updrafts which carry monopolar positive charge aloft leaving negative space charge in a central or lower part of the cloud. Precipitation and downdrafts can also transport negative space charge downward. The updrafts as downdrafts further contribute to the electrification of the cloud particles. Lightning generally occurs near the location of these intense updrafts and downdrafts.
Lightning begins with a an initial electrical breakdown (pulse) followed by leader channels from which a series or channel branches grow within a cloud forming a comprehensive branch channel structure. For IC lightning, the channel structure remains within the cloud. A CG discharge occurs when one or more branches extend from a cloud to the ground. The leader channel propagates in steps to the ground. When the leader channel is about 100 meters from the ground, a streamer propagates up from the ground to meet the stepped leader. When the two meet, a continuous channel of ionized air is formed from the cloud to the ground. At this point a large current flows from the ground to the cloud which is known as a return stroke.
Typical lightning detection systems, such as the Lightning Location and Protection (LLP) system used in the National Lightning Detection Network (NLDN), operate to detect CG return strokes. Generally, the return stroke associated with a CG discharge is many times larger than for IC discharges. Thus, such a system typically will not detect IC lightning. Also, the system assigns a location to the discharge corresponding to a position on the around and provides no information with respect to the stroke origin which may be tens of miles distant.
A further disadvantage of prior single sensor systems is susceptibility to RF noise. Since Very Low Frequency (VLF) signals are targeted for detection, systems for detecting CG return strokes can provide a general bearing of a storm using well known crossed loop technology, but are subject to gross errors with response to distance. In particular, single sensor VLF systems determine He distance of a lightning stroke from the stoke intensity, but stroke intensities can vary by two or three orders of magnitude. Thus it will be appreciated that these VLF systems do not have the capability to accurately determine lightning range from a single observation station.
Another disadvantage of known systems stems from display limitations with respect to lightning position More particularly, a single dot on a display typically represents a complete lightning flash. However, a lightning flash can extend for tens of kilometers from an initial leader. Thus a display of dots may provide a general area containing lightning discharges, but does not provide an accurate representation of the location of the source of the atmospheric disturbance. The source of the lighting is generally the area presenting the most severe aviation hazards, such as hail, icing, turbulence and microbursts.
A still further drawback of some systems is that in operation, the systems detect and process energy from many parts of the lightning channels of lightning strokes and the multitude of pulses from each stroke, thus requiring a tremendous processing capability. Such systems are complex and expensive.
With respect to known VLF single station lightning detection systems, there are considerable limitations associated therewith due to the inherent variation in lightning stroke discharge amplitudes. For example, a lightning channel structure includes a tremendous horizontal and vertical span radiating energy throughout. This produces polarization errors for azimuth and distance determination. Lightning discharges vary in intensity as much as three orders of magnitude, thus precluding accurate distance determination based on detected discharge intensity. Also, IC and CG discharges have different characteristics. These factors contribute to the problems of fabricating an accurate lightning detection system including the ability to distinguish between IC and CG lightning.
Often during thunderstorms, intense downdrafts, known as microbursts, follow lightning producing updrafts. Microbursts pose a threat to aircraft, especially immediately after take off and prior to landing where an aircraft is especially vulnerable. A further danger to aircraft results as a microburst approaches ground level and air flows horizontally creating a wind shear region possibly resulting in stalling the aircrafts and losing lift. In fact several hundreds of deaths have occurred in airplane crashes over the past few decades due to intense downdrafts, or microbursts and resulting wind shear. Since lightning generally begins near the locations of intense updrafts and downdrafts, the early detection of microbursts is critical in averting such disasters. This situation has been partially addressed by the Federal Aviation Administration (FAA) which has responded by situating weather radars, such as Terminal Doppler Weather Radars (TDWR), at various major airports across the United States. These radars measure the radial velocity of raindrops towards and away from the radar and infer air motions therefrom. However, despite the considerable cost, in the neighborhood of several millions of dollars, Doppler weather radars have limitations. For instance, if rainfall is vertical and the radar is scanning near the horizon, no radial velocity is detected, thus not detecting a possible downdraft. Doppler radars operate to detect outflow air having rain drops therein. Further disadvantages of weather radars are slow volume scans, for example up to three minutes to obtain one picture, performance degradations due to ground clutter, and significant cost.
Known lightning detection systems do not provide a way to determine potential microburst locations since there is no known correlation between CG discharges and microbursts. It will be appreciated by one skilled in the art that a VLF system detecting signals having wavelengths on the same scale as lightning channels is not well adapted for microburst prediction which requires defining lightning in scales of hundreds of meters. A VLF system detects a CG return stroke emitting VLF energy having a wavelength in the order of 10, but can rarely, if at all, detect the shorter stepped structure of IC lightning rich in HF and VHF radiation from which microbursts can be predicted.
Other technologies are currently being developed and exist to detect windshear conditions, such as laser, and Infra Red (IR) and Doppler radar based systems. While those technologies may be successful in ascertaining microbursts that have already developed, it is unlikely that event prediction will be attainable. Hazardous weather warnings would be in the vicinity of a few minutes, or seconds, and thus possibly not sufficient for an aircraft to avoid the danger.
In view of the above, a low cost total lightning warning system is desired which detects all lightning discharges and provides an early warning of lightning, accurate location including direction and range of the source of lightning, lightning type, and the location of potential microbursts.